Developmental changes in genetic and environmental effects have often been ignored in the analysis of family data. Longitudinal studies of the genetics of development have been hampered by lack of adequate mathematical models for the developmental process. We shall examine theoretical aspects of developmental change to provide parsimonious models for the cumulative effects of genes and environment from childhood to adulthood. Mathematical analysis and computer simulations will compare the ability of different types of study to resolve theories of developmental change. Analysis of existing cross-sectional data on nuclear families, adoptions, extended pedigrees, kinships of twins and their relatives will detect the interaction of genetic differences with age. Longitudinal data on twins and their parents will be used to test alternative hypotheses of behavioral and physiological development.